1. Field of the Invention
The present invention relates generally to a method of depositing thin films. In particular, the invention concerns a method of growing elemental metal thin films by Atomic Layer Deposition (ALD) using a boron compound as a reducing agent.
2. Description of the Related Art
The integration level of components in integrated circuits is increasing, producing a need for smaller components and interconnects. Design rules are dictating a feature size less than or equal to 0.2 μm. This makes complete film coverage on deep vias difficult to obtain.
Integrated circuits contain interconnects that are conventionally made of aluminum. Today, copper is replacing aluminum because it has lower electrical resistance and better electromigration resistance than aluminum.
Chemical Vapor Deposition (CVD) has been commonly used to produce metal films. In CVD, the source materials are typically fed into a reaction space together, where they react when brought into contact with a hot substrate. Thus, the growth rate of the metal film depends in part upon the concentration of the different source materials in the reaction space. Additionally, the temperature of the substrate affects the rate of deposition. In thermal CVD a single source chemical can be thermally decomposed near the substrate.
Atomic Layer Deposition (ALD) is an advanced alternative to CVD. The ALD method is based on sequential self-saturating surface reactions and has been described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. Source chemicals are pulsed into the reaction chamber in an inert carrier gas. The pulses of source chemical can be separated from each other by a purging flow of inert gas. The separation of the source chemicals and the proper choice of source chemicals prevents gas-phase reactions between gaseous reactants and enables self-saturating surface reactions. This allows for film growth without strict temperature control of the substrate or precise dosage control of the reactants. Surplus reactants and byproducts are removed from the chamber, such as by a purging flow of inert gas, before the next reactive chemical pulse is introduced. Undesired gaseous molecules are effectively removed from the reaction chamber by keeping the gas flow speeds high. The purging gas pushes the extra molecules towards the vacuum pump that is used to maintain a suitable pressure in the reaction chamber. Thus, ALD provides for rapid, uniform, controlled film growth.
While ALD has been used to produce both elemental and compound thin films, there are a number of drawbacks to the methods that have been used. Thus, a need exists for improvements in the production of metal thin films.